The present invention relates to waveguide filters and in particular, but not limited to waveguide filters for RF waves.
Radio transmitters and receivers require filters to remove or suppress unwanted frequencies from being transmitting or received. The transmitter portion of the radio may generate frequencies which will interfere with the radio system, or which may be prohibited by the radio frequency spectrum governing body. The receiver may need to suppress unwanted signals at different frequencies generated by the transmitter, or received from an external source, which would adversely affect the performance of the receiver.
At millimeter-wave frequencies sources of unwanted frequencies include the local oscillator frequency, image frequencies from the mixer, and the transmitter frequencies (in the case of the receiver). The frequencies generated by the mixer and the local oscillator are functions of the selected radio architecture. The closer the oscillator frequency (or its harmonics) is to the transmitter frequencies, the more difficult it is to remove the undesired frequency. However, wider spaced frequencies may result in more complex circuitry resulting in a more expensive radio implementation. A small separation between the transmit and receive frequencies can result in unwanted high power transmit frequencies leaking into the receiver. The separation between the transmit and receive frequencies is usually specified by the licensing bodies and the system operators. The radio designer may not have control over this specification.
To suppress the unwanted frequencies below an acceptable power level, a filter element is required in the signal path. The filter element discriminates between the desired and undesired frequencies based on the wavelengths of the signals. At millimeter-wave frequencies the difference between the wavelengths is very small, resulting a in very high manufacturing tolerances.
A common millimeter-wave filter is based on the metal rectangular waveguide, an example of which is shown in FIGS. 1a and 1b. The waveguide 1 comprises a series of resonant cavities 3 separated by partitions S. Each partition has an aperture or iris 7 to permit coupling of electromagnetic energy between the resonator cavities 3. Adjustable posts or tuning screws 9 extend into each cavity to provide a means of adjusting the resonant frequency of each cavity which is dependent on the cavity volume. A rectangular waveguide is used for its low loss characteristics. The resonant elements which when combined generate the filter response are formed in the waveguide mainly through the use of irises and posts. The resonant sections 3 are formed from lengths of waveguide multiples of one half wavelength long, with the size and placement of the irises or posts determining the coupling between the resonators and hence the frequency behaviour of the filter.
For the filter to discriminate between closely spaced frequencies, the physical dimensions must be extremely accurate. In practice it is difficult and costly to achieve the required dimensional accuracy. Historically many millimeter-wave applications did not require high volume production, and thus the investment to achieve the necessary accuracy was not warranted. Adjustable tuning screws were included in the design and after manufacture and assembly each filter was individually tuned, manually or automatically, to achieve the desired frequency response.
The use of tuning screws results in increased costs when compared to machined or case filters due to the more complicated assembly and tuning steps in the manufacturing process. Examples of these filters are disclosed in the publications of commercial millimeter-wave waveguide filter or diplexer component manufacturers, such as Microwave Development Company Inc., Lark Engineering, or XandL Microwave Inc.
A typical metal insert or E-plane filter is shown in FIGS. 2a and 2b. The waveguide housing 10 is split into two sections 12, 14, along the middle of the long dimension. The metal insert piece or septum 16 behaves as a series of posts when the filter is assembled. The accuracy of fabrication of the metal insert piece, which is normally etched and has a dimensional accuracy of xc2x10.1 mil (i.e. 0.0001 inches) is sufficient to ensure that there is no significant affect on the filter response at millimeter-wave frequencies. The frequency of operation of the filter is therefore set by the accuracy of the depth xe2x80x9cdxe2x80x9d of the waveguide housing, as shown in FIG. 2a. 
A benefit of the metal insert filter is that the same housing can be used for different filters at different frequencies. Only the metal insert piece needs to be changed, and there is not a significant setup charge for changes in the metal insert piece.
To achieve the tight filtering requirements for closely spaced local oscillator and transmit frequencies, or between transmit and receive frequencies, for LMDS (Local Multipoint Distribution Service) radio applications the accuracy of the depth of the waveguide housing needs to be better than +/xe2x88x921 mil. This level of accuracy can be achieved with quality machining, but is expensive to achieve for volume production.
According to one aspect of the present invention, there is provided an E-plane waveguide comprising a housing having opposed walls and defining a waveguide channel therebetween, at least two elements spaced apart in a direction along the waveguide channel and disposed between and spaced from said opposed walls and defining a resonant cavity therebetween with said waveguide channel, wherein at least one of said walls has first and second regions separated in a direction along said waveguide channel, at least one of said first and second regions being disposed opposite the aperture formed between said elements, and a third region between said first and second regions, and wherein said first and second regions protrude into said waveguide channel relative to said third region, and the spacing between said first and second regions in a direction along said waveguide channel is less than half a wavelength of the resonant frequency of said resonant cavity.
Advantageously, the inventors have found that providing the cavity wall with a structured surface having protrusions which extend into the cavity can significantly reduce the frequency shift of a waveguide filter due to errors in the width of the waveguide channel caused by the manufacturing process.
In one embodiment, at least one of the first and second regions comprises a discrete protrusion extending into the cavity relative to the third region.
In one embodiment, at least one dimension of the first region or protrusion is different from at least one corresponding dimension of the second region or protrusion. Advantageously, this feature may promote statistical variations in the manufacturing error by increasing the chance that protrusions having different dimensions will be subjected to slightly different manufacturing process conditions.
In one embodiment, the maximum dimension of the or each discrete protrusion transverse to a line perpendicular to the plane of the wall from which the or each protrusion extends is less than or equal to the spacing between itself and the other region or protrusion. Advantageously, this feature allows the protrusions to be rotated, for example for the purpose of adjusting their height and is particularly advantageous in the design and testing of a template of a waveguide housing section, which may be used to produce a cast or mold.
According to another aspect of the present invention, there is provided a housing section for an E-plane waveguide comprising a first wall and a second wall for forming part of said waveguide, said first wall adjoining said second wall, and in use, said first wall spacing said second wall from a septum of said E-plane waveguide, and wherein the side of said second wall which, in use, faces the waveguide channel, has said first and second regions spaced apart in a direction along said housing section, and a third region being positioned between said first and second regions, said first and second regions protruding from said second wall relative to said third region, and wherein the spacing between said first and second regions in a direction along the waveguide housing section is such that resonance of the frequency to be resonated within a cavity of an E-plane waveguide formed by said housing section is prevented.
According to another aspect of the present invention, there is provided a cast for manufacturing a waveguide housing section, the cast of having a form adapted to form a waveguide housing section as described herein.
According to another aspect of the present invention, there is provided a method of forming a cast for manufacturing a waveguide housing section, comprising the steps of: forming a template waveguide housing section, the waveguide housing section comprising a waveguide channel wall having a plurality of projections extending from the wall and being spaced apart along the wall, and forming a cast which conforms to the shape of said wall containing said projections.
According to another aspect of the present invention, there is provided a waveguide filter having a opposed walls and defining a channel therebetween, wherein the surface of at least one of the walls defining the channel defines a plurality of projections spaced apart in a direction along the length of the channel, wherein the spacing between adjacent projections is less than half a wavelength of the RF signal intended to be passed by the filter.
According to another aspect of the present invention there is provided a resonator for resonating an RF wave having a predetermined frequency and wavelength, the resonator comprising a resonant cavity having a wall, the wall having first, second and third regions, the third region being positioned between the fist and second regions, wherein the first and second regions protrude into the cavity relative to the third region wherein the distance between the first and second regions is less than half said predetermined wavelength.
In one embodiment, at least one of said first and second regions comprises a discrete protrusion extending into the cavity relative to the third region.
One embodiment further comprises one or more spaced apart further regions which protrude into said cavity relative to said third region, wherein the closest separation between adjacent projecting regions is less than half said predetermined wavelength.
In one embodiment, the closest separation between at least two projecting regions is one third of said predetermined wavelength or less.
According to the present invention there is further provided a resonator for resonating an RF wave having a predetermined frequency and wavelength comprising a resonant cavity having at least two spaced apart projections on the same side of said cavity, the ends of said projections defining part of the wall of said cavity, the distance between said projections being less than half said predetermined wavelength.
According to the present invention, there is also provided a waveguide comprising a channel for receiving electromagnetic waves and having opposed walls, the inside surface of at least one of said opposed walls being defined by at least two projections, spaced apart in a direction along the length of said channel, wherein the distance between the projections in a direction along the length of the channel is less than half the predetermined wavelength of electromagnetic waves to be passed through said waveguide.
In one embodiment the maximum dimension across the end of at least one projection defining the wall of the channel, in direction along the length of the channel is less than a half of said predetermined wavelength.
In one embodiment a portion a portion of each of said opposed walls of said channel are defined by at least two said projections.
One embodiment further comprises an element between said opposed walls and extending in a direction along the length of said channel and defining an aperture therethrough, the dimension of said aperture in direction along the length of said channel defining said predetermined resonant wavelength of said RF wave, at least a portion of at least one of said projections being positioned opposite said aperture.
In one embodiment the element comprises a plurality of apertures positioned successively along the length of said channel, a dimension of each aperture along the length of said channel defining a resonant wavelength of an RF wave to be passes through said waveguide.
In one embodiment at least one aperture defines a different resonant wavelength to another said aperture.